Fix #70: Use %w formatting directives when fmt.Error'ing an error.

[constantoine]

As explained in #70, not using %w formatting directive in some places makes it non-trivial for the user to make use of the error in a meaningful way (errors.Is or errors.As), outside of string matching. This is particularly painful for errors returned by custom type's XXXUnmarshaler

This PR aims to change the remaining fmt.Errorf calls where %w directive isn't used but an error is passed while adding no new code.

[constantoine]

Hi @jamietanna I hope the ping is not inappropriate :)

Do those changes (as well as the rationale behind them) sound acceptable to you?

[jamietanna]

Pings are not recommended 😅

I have seen this and I do agree with it, thanks for fixing it!

I hadn't got round to replying here, and would like to merge this alongside some other runtime changes, but my focus recently has been trying to get oapi-codegen itself ready for a release, which is slow work 🙃

Will try to get to this soon!